1. Field of the Invention
The present invention relates to a package for accommodating a semiconductor element having a heat release structure with good heat releasing properties and a semiconductor device using the same. Furthermore, the present invention relates to a heat releasing member and a package for accommodating a semiconductor element having a heat release structure with good heat releasing properties, and a semiconductor device using the same.
2. Description of the Related Art
Conventionally, a semiconductor-element-accommodating package for accommodating a semiconductor element is generally formed of an insulating frame made of an electric insulating material such as aluminum oxide sintered substances, mullite sintered substances, or glass ceramic sintered substances, a heat releasing member on which a semiconductor element is mounted for satisfactorily releasing heat generated during operation thereof to the air and that is made of an alloy material of copper and tungsten or an alloy material of copper and molybdenum, and a lid. The insulating frame is provided so as to surround a portion on the upper surface of the heat releasing member on which the semiconductor element is mounted, and a plurality of wiring conductors made of tungsten, molybdenum, manganese, copper, silver or the like are attached and extended from the inside to the external surface of a recess formed by the insulating frame and the heat releasing member. Then, the semiconductor element is adhered and fixed onto the portion on the upper surface of the heat releasing member on which the semiconductor element is mounted via an adhesive such as glass, resin, or brazing materials, and each electrode of the semiconductor element is electrically connected to the wiring conductor via a bonding wire. Then, sealing resin such as epoxy resin is injected to the recess formed by the insulating frame and the heat releasing member so as to seal the semiconductor element, and thus a semiconductor device is obtained as a product. This semiconductor device may be mounted on an external heat releasing plate by screwing in order to improve the heat release efficiency.
Such a package for accommodating a semiconductor element provided with a heat releasing member that is made of an alloy material of tungsten and copper or the like has attracted attention as a package for accommodating a semiconductor element on which a high heat generating semiconductor element such as power ICs or high frequency transistors, because the heat conductivity of the heat releasing member is high and the thermal expansion coefficient of the heat releasing member is approximate to the thermal expansion coefficient of silicon or gallium arsenic, which is a constituent material of the semiconductor element, a ceramic material, which is used as a constituent material of the package, or the like.
In recent years, there is a demand for a heat releasing member having a heat conductivity of 300 W/m·K or more, because of a recent increase of the amount of generated heat with increasing integration of power ICs or high frequency transistors. However, the heat conductivity of the heat releasing member made of an alloy material of tungsten and copper or an alloy material of molybdenum and copper as described above is about 200 W/m·K, which is low for this requirement, and thus the heat releasing properties are becoming insufficient.
On the other hand, it has been proposed to use a heat releasing member made of a composite material in which tungsten and copper constitute a matrix. Furthermore, for example, in Japanese Unexamined Patent Publication JP-A 9-312361, it also has been proposed to use a heat conductive substrate made of a composite material in which high heat conductive layers of copper or a copper alloy and low thermal expansion layers made of a Fe—Ni alloy are laminated alternately, and the high heat conductive layers sandwiching the low thermal expansion layer are continuous via a plurality of through-holes formed in the low thermal expansion layer.
However, in a package for accommodating a semiconductor element using the heat release member made of the composite material in which tungsten and copper constitute a matrix, tungsten has a low heat conductivity and a low thermal expansion coefficient, and copper has a high heat conductivity and a high thermal expansion coefficient, so that the heat conductivity and the thermal expansion coefficient of the heat release member can be increased as the content of copper is increased. However, when the content of copper is increased in order to improve the heat conductivity, the difference in the thermal expansion coefficient between the semiconductor element and the heat release member is increased, so that the semiconductor element cannot be joined to the heat releasing member firmly.
When the heat conductive substrate made of a composite material including high heat conductive layers of copper or a copper alloy and low thermal expansion layers made of a Fe—Ni alloy is used, in general, the Fe—Ni alloy has a low heat conductivity (e.g., about 16 W/m·K in the case of a Fe—42Ni alloy), and the heat transfer properties in the thickness direction of the substrate is low.
In addition, in the case of the composite material in which high heat conductive layers of copper or a copper alloy and low thermal expansion layers made of a Fe—Ni alloy are laminated alternately, and the high heat conductive layers sandwiching the low thermal expansion layer are continuous via a plurality of through-holes formed in the low thermal expansion layers materials having different thermal expansion coefficients are disposed in a complex manner, so that the substrate may be bent significantly during heating.
Furthermore, in the package for accommodating a semiconductor element using a heat releasing member made of this composite material, copper expands and is plastically deformed at high temperatures when assembling the package, so that the heat releasing member does not return to the original state and the surface of the heat releasing member becomes rough.
In general, the surface roughness of the heat releasing member should be Ra≦30 μm, where Ra is an arithmetic mean roughness, in order to prevent reduction of the bond strength the heat releasing member and the semiconductor element due to void production in an adhesive when the semiconductor element is adhered and fixed to the heat releasing member via the adhesive such as glass, resin or brazing materials. Therefore, when a heat releasing member made of this composite material is used, the surface is subjected to smoothing by polishing in order to make the surface roughness be Ra≦30 μm, where Ra is the arithmetical mean roughness. However, in the package on which an insulating frame is attached so as to surround the mounting portion for a semiconductor element, the mounting portion cannot be polished.
However, when the surface roughness of the heat releasing member is significantly small, the area in which the heat releasing member and the sealing resin are in contact with each other is decreased, and the anchoring effect of the sealing resin to the heat releasing member is reduced, so that the bond strength between the heat releasing member and the sealing resin is degraded. Thus, peeling may occur at the interface between the heat releasing member and the sealing resin.
Similarly, the sealing resin may cover the entire upper surface of the heat releasing member and the insulating substrate and go up to the side face of the heat releasing member. In this case as well, when the surface roughness on the side face of the heat releasing member is significantly small, the area in which the heat releasing member and the sealing resin are in contact with each other is decreased, and the anchoring effect of the sealing resin to the heat releasing member is reduced, so that the bond strength between the heat releasing member and the sealing resin is degraded. Thus, peeling may occur at the interface between the heat releasing member and the sealing resin.